Optical depolarizers and DGD generators based on optical delay

ABSTRACT

Techniques and devices for depolarizing light and producing a variable differential group delays in optical signals. In one implementation, an input optical beam is split into first and second beams with orthogonal polarizations. One or two optical reflectors are then used to cause the first and second optical beams to undergo different optical path lengths before they are recombined into a single output beam. An adjustment mechanism may used implemented to adjust the difference in the optical path lengths of the first and second beams to produce a variable DGD. When the depolarization of light is desired, the difference in the optical path lengths of the first and second beams is set to be greater than the coherence length of the input optical beam.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of and claims priority toU.S. application Ser. No. 11/616,264, filed Dec. 26, 2006, U.S. Pat. No.7,535,639, which is a divisional of U.S. application Ser. No.10/418,712, filed on Apr. 17, 2003, now U.S. Pat. No. 7,154,659, whichclaims benefit of U.S. Provisional Application Ser. No. 60/413,806,filed Sep. 25, 2002, and U.S. Provisional Application Ser. No.60/373,767, filed Apr. 18, 2002.

The disclosures of the above-referenced applications are incorporated byreference as part of the disclosure of this application.

BACKGROUND

This application relates to optical devices, and in particular, tooptical depolarizers and devices for generating differential groupdelays (DGDs) and their applications.

Optical depolarizers are optical devices for reducing the degree ofoptical polarization of an input optical beam or randomizing the inputpolarization. Applications for such depolarizers include but are notlimited to optical networks, test & measurement, and sensorapplications. In an optical network application, for example, adepolarizer may be used to eliminate polarization sensitivity of Ramanamplifiers. In test and measurement systems, depolarizing the outputbeam from a source laser may be used to eliminate polarizationsensitivity of many test instruments.

Generation of variable DGDs has applications in optical communicationsystems and devices where polarization-mode dispersion (PMD) is present.

SUMMARY

This application includes techniques and devices to depolarize light andto produce a desired differential group delay in optical signals. Ingeneral, an input optical beam is split into first and second beams withorthogonal polarizations. One or two optical reflectors are then used tocause the first and second optical beams to undergo different opticalpath lengths before they are recombined into a single output beam. Anadjustment mechanism may be implemented to adjust the difference in theoptical path lengths of the first and second beams to produce a variableDGD. When the depolarization of light is desired, the difference in theoptical path lengths of the first and second beams is set to be greaterthan the coherence length of the input optical beam.

In one exemplary implementation, a device of this application mayinclude a first polarization beam splitter (PBS), a second PBS, and areflector arranged to form an optical system. The first PBS ispositioned to receive an input optical beam with a coherent length andto split the input optical beam into a first beam and a second beampolarized orthogonal to the first beam. The reflector is positioned toreflect the first beam to the second PBS to cause an optical pathdifference between the first and the second beams at the second PBS tobe greater than the coherent length. The second PBS is positioned toreceive and combine the first and the second beams to produce an outputbeam.

In another exemplary implementation, a device of this application mayinclude a polarization beam splitter (PBS) to receive an input opticalbeam with a coherent length and to split the input optical beam into afirst beam and a second beam polarized orthogonal to the first beam. Thedevice also includes first and second reflectors, first and secondpolarization elements. The first reflector is positioned relative to thePBS to reflect the first beam back to the PBS. The first polarizationelement is located between the first reflector and the PBS to rotate apolarization of a reflection of the first beam at the PBS to beperpendicular to the first beam when initially exiting the PBS. Thesecond reflector is positioned relative to the PBS to reflect the secondbeam back to the PBS. The second polarization element is located betweenthe second reflector and the PBS to rotate a polarization of areflection of the second beam at the PBS to be perpendicular to thesecond beam when initially exiting the PBS. The first and the secondreflectors are positioned to produce a difference in optical paths ofthe first and the second beams upon being reflected back to the PBS thatis greater than the coherent length of the input optical beam.

The above and other devices of this application may operate asdepolarizers. In addition, such devices may also be applied to produce afixed pure first-order differential group delay (DGD). Pure first orderDGD can have important applications in compensating for polarizationmode dispersion. The DGD devices may be designed with low fabricationcost and compact size in order to compete with PM fibers. Thepolarization insensitive version of such a device may also be used as apassive bandwidth limiter.

These and other implementations, features, and associated advantages arenow described in detail with reference to the drawings, the detaileddescription, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exemplary device for depolarization or generating avariable DGD, which includes two polarization beam splitters (PBS) andan optical reflector such as a prism reflector.

FIG. 2A shows an exemplary polarization-insensitive device of thisapplication.

FIG. 2B illustrates orientations of polarizations in the device in FIG.2A.

FIGS. 3A and 3B show another exemplary polarization-insensitive deviceand the orientation of an input polarization beam displacer with respectto the input state of polarization.

FIG. 4 shows one implementation of a device having a polarization beamsplitter and two optical reflectors.

FIG. 5 shows the orientation of quarter waveplates used in the device inFIG. 4 for rotating optical polarization of each reflected beam.

FIG. 6 shows a modified device based on the designs in FIGS. 4 and 5 byusing a Faraday reflector in one reflection optical path and a fiber toprovide a long optical delay relative to another reflection opticalpath.

FIG. 7A shows a device based on the designs in FIGS. 4 and 5 with a dualfiber collimator and a prism polarization beam combiner in the inputpath.

FIG. 7B shows relevant states of polarization in the device in FIG. 7A.

FIGS. 8 and 9 show two additional exemplary devices based on the designin FIG. 4.

FIG. 10 shows yet another design for implementing depolarizers or DGDgenerators.

DETAILED DESCRIPTION

The techniques and devices of this application split an input opticalbeam at an input location into first and second beams with orthogonalpolarizations. At least one reflector is used to reflect the first inputbeam along a path different from the second beam to produce a differencein optical path lengths of the two beams at a common location where theyare recombined into a single output beam.

FIG. 1 shows one implementation of a depolarizer 100 which includes twopolarization beam splitters (PBSs) 110 and 120 and an optical reflector130 such as a prism reflector. The first PBS 110 is used as an inputport to receive input light 101 and to split input light into twoorthogonal linear polarizations 111 and 112. The transmittedpolarization 111, e.g., the P polarized light, is directed to the prismreflector 130. The reflected polarization 112, e.g., the S polarizedlight, is directed to the second PBS 120. The transmitted P polarizedlight 111, after reflected by the prism reflector 130, is directed tothe second PBS 120. The second PBS 120 combines the S and P polarizedbeams 112 and 111 to produce the output beam 121. When operated underproper conditions, this output beam 121 is depolarized.

Notably, the distance between the prism reflector 130 and the two PBSs110 and 120 may be sufficiently long to be greater than the coherentlength of the input optical beam 101 so that the S and P polarized beams112 and 111 received by the second PBS 120 are no longer coherent witheach other. This condition allows the output light 121 from the secondPBS 120 to be effectively depolarized. For a linear input beam 101, theinput polarization should be at 45 degrees with respect to the passingpolarization axis of the first PBS 110 to evenly split the input powerbetween two output beams 111 and 112 of the first PBS 110.

In one implementation, an input fiber may be used to direct the inputbeam 101 to the first PBS 110. Accordingly, an output fiber may be usedto receive the output beam 121 from the second PBS 120. The input fibermay be polarization maintaining (PM) and the input light 101 is oriented45° from the passing axis of the first PBS 110. Under this condition,the linearly polarized input light 101 is split into “s” and “p”components 112 and 111 with equal power levels. Assuming the PBS 110reflects the “s” component 112 and transmits the “p” component 111, the“p” component 111 undergoes a longer optical path than the “s” componentwhen they reach the second PBS 120. In order to achieve effectivelydepolarization, the optical path difference between the two components111 and 112 should be larger than the coherence length of the lightsource for producing the input light 101. In comparison with a typicalbirefringent-crystal-based Lyot depolarizer, this device 100 has theadvantage of smaller size because of the double pass free-space design.In addition, the cost of the device 100 can be low because nobirefringent crystal is required. Table I shows the minimum devicelength for light source with different linewidth. The length of thisdepolarizer may be significantly shorter than a typical Lyotdepolarizer, e.g., as much as 10 times less that that of a singlesection Lyot depolarizer.

TABLE I Length of Lyot depolarizer Linewidth Coherent length Length ofGP depolarizer (Δn = 0.2) 1 nm 2.4 mm 1.2 mm 12 mm 0.1 nm 24 mm 12 mm120 mm 0.01 nm 240 mm 120 mm 1200 mm

When the input fiber that feeds the input light 101 to the PBS 110 isimplemented with a single mode fiber, the device 100 may be operated asa differential group delay line (DGD) for PMD compensation becausedifferent polarization components undergo different optical path delays.As a variable DGD generator, it is not necessary that the difference inthe optical path lengths in the device 100 be greater than the coherencelength of the input light. The relationship between the minimum devicelength (excluding lengths of PBS and reflection prism) and DGD is listedin Table II.

TABLE II Equivalent optical DGD path length Device length 10 ps 3 mm 1.5mm 25 ps 7.5 mm 3.75 mm 50 ps 15 mm 7.5 mm 100 ps 30 mm 15 mm 200 ps 60mm 30 mm

The design 100 shown in FIG. 1 may be used to construct compact DGDcomponents. In addition, because the light beams propagate in the airbetween the PBSs 110 and 120 and the prism reflector 130 within thedepolarizer 100 and the air has negligible dispersion, the device 100may operate to produce a pure first order DGD.

The depolarizer 100 described in FIG. 1 is sensitive to the inputpolarization. In many applications, polarization insensitivity may bedesirable. FIGS. 2A, 2B, 3A, and 3B illustrate two configurations thatcan eliminate the polarization sensitivity of the device in FIG. 1.

FIG. 2A shows the first polarization-insensitive depolarizer 200according to one implementation. An input polarization beam displacer(PBD) 210 is placed in the input of the first PBS 110 to separate twopolarization components into two parallel paths 211 and 212 into thefirst PBS 110. A polarization rotator 218 such as a half wave plate isplaced in one of the parallel paths to rotate the polarization in thatpath by 90°. Consequently, the two parallel input beams 211 and 212 havethe same linear polarization when entering the first PBS 110. The inputPBD 210 is oriented in such a way that the linear polarization of thetwo input beams 211 and 212 is 45° from the passing axis of the firstPBS 110, as shown in FIG. 2B. As a result, each beam is split into twobeams by the first PBS 110. The “p” component transmitting through thePBS 110 goes through a longer optical path through the reflector 130before combining with the “s” component at the second PBS 120. Finally,the two beams are combined by a second, output PBD 220 to produce adepolarized output beam 221. A second half wave plate 228 is placed inone of the parallel paths between the PBS 120 and the PBD 220 to rotatethe polarization in that path by 90°. The PBD 220 is orientedcomplementarily from the first input PBD 210 to allow two parallel inputbeams with orthogonal polarizations to be combined into the singleoutput beam 221. This output beam 221 may then be directed into theoutput fiber in a fiber system.

The PBDs 210 and 220 may be implemented in various configurations. Forexample, a properly-cut birefringent crystal, such as calcite, may beused to separate the ordinary and extraordinary beams with orthogonalpolarizations as parallel output beams. The ordinary polarizationtransmits straight through while the extraordinary transmits through thecrystal at an angle with respect to the ordinary beam and emergesparallel to the ordinary beam.

In a different configuration 300 shown in FIG. 3A, a first PBD 210 isused to receive an input beam 101 and separates the beam 101 upontransmission into two parallel beams 211 and 212 with orthogonalpolarizations. The PBD 210 is so oriented that the two orthogonalpolarizations are +/−45° from the passing axis of the PBS as shown inFIG. 3B. Consequently, each beam is split into “s” and “p” components.Similarly, the “p” component travels along a longer optical path throughthe reflector 130 before being combined with the “s” component at thesecond PBS 120. Finally, the two beams are combined by a second PBD 220oriented complementarily from the first PBD 210 to produce thedepolarized output beam 221. In this configuration, the half wave plates218 and 228 used in FIG. 2A are eliminated to reduce the cost of thedevice and to reduce its wavelength sensitivity introduced by the halfwave plates.

A pure first order DGD may be used in the polarization mode dispersioncompensation. However, the DGD device should be compact and can bemanufactured at a low cost in order to compete with a PM fiber DGDdevice. The above depolarizers may be used to produce such pure firstorder DGDs for various applications.

Notably, an adjustment mechanism may be implemented in the above andother exemplary devices of this application to adjust the spacingbetween the prism reflector 130 and the PBSs 110 and 120 to producedifferent or variable first order DGDs. This adjustment mechanism may beimplemented by, e.g., engaging the reflector 130 to a movable element205 that moves its position to change the position of the reflector 130in response to a control signal. Alternatively, the two PBSs 110 and 120may be engaged to the movable element 205 to move relative to thereflector 130.

The following sections of this application describe additional exemplarydesigns of optical depolarizers, including, among others, designs with along coherence length. Notably, FIGS. 4. 6, 7A, 8 and 9 show exemplaryimplementations of a different device configurations for either opticaldepolarization or generation of variable DGDs by using two separatereflectors to form two separate reflection optical paths.

This type of design uses a polarization beam splitter (PBS) 410 toreceive an input optical beam with a coherent length and to split theinput optical beam into a first beam and a second beam polarizedorthogonal to the first beam. First and second reflectors 420 and 430,first and second polarization elements 441 and 442 are used to form twodifferent optical reflection arms. The first reflector 430 is positionedrelative to the PBS 410 to reflect the first beam back to the PBS 410.The first polarization element 441 is located between the firstreflector 430 and the PBS 410 to rotate a polarization of a reflectionof the first beam at the PBS 410 to be perpendicular to the first beamwhen initially exiting the PBS 410. The second reflector 420 ispositioned relative to the PBS 410 to reflect the second beam back tothe PBS 410. The second polarization element 442 is located between thesecond reflector 420 and the PBS 410 to rotate a polarization of areflection of the second beam at the PBS 410 to be perpendicular to thesecond beam when initially exiting the PBS 410. The first and the secondreflectors 430 and 420 are positioned to produce a difference in opticalpaths of the first and the second beams upon being reflected back to thePBS 410. When operated as an optical depolarizer, this difference is setto be greater than the coherent length of the input optical beam.

FIG. 4 shows one implementation of a depolarizer 400 having apolarization beam splitter 410 and two optical reflectors 420 and 430.The polarization beam splitter (PBS) 410, two polarization rotators 441and 442, two mirrors (reflectors) 420 and 430 and a prism reflector 450are arranged as illustrated. The input light 101 may be delivered to theinput facet of the PBS 410 with the input SOP oriented a degrees fromthe passing axis of the PBS. An input fiber, made of a polarizationmaintaining (PM) fiber having one polarization axis aligned at a degreesfrom the passing axis of the PBS, may be used to deliver the input lightto the PBS 410. Each of the polarization rotators 441 and 442 may beeither a 45-degree Faraday rotator or a quarter-wave plate.

Under this input condition, the linearly polarized input light 401 issplit by the PBS 410 into two orthogonally polarized beams 412 and 411:the “s” and “p” components. Assuming the PBS 410 reflects the “s”component 412 and passes the “p” component 411, the “p” component 411goes through a longer optical path than the “s” component 412. Afterreflection from the mirrors 420 and 430, the “s” component 412 becomes“p” and the “p” components 411 becomes “s” so that both components aredirected towards the prism reflector 450 by the PBS 410. In order toachieve effective depolarization, it is desirable that the optical pathdifference between the two components be larger than the coherencelength of the light source that produces the input light 401. Incomparison with a typical Lyot depolarizer, this device 400 has theadvantage of smaller size because of the double pass free-space design.In addition, the cost of the device 400 is also lowered because nobirefringent crystal is required. Table III shows the minimum devicelength for light source with different linewidth. As indicated, thelength of GP's depolarizer is 10 times less that that of a singlesection Lyot depolarizer.

In this implementation, the powers of the “s” and “p” components 412 and411 should also be equal at the output in order to be an effectivedepolarizer. Assuming the transmission coefficients of the “s” and “p”components are T_(s) and T_(p) respectively, the orientation angle α ofthe input SOP should be:α=tan⁻¹(T _(s) /T _(p)).Note that if quarter waveplates are used as the polarization rotator 441or 442, the relative orientation angle of the waveplates should be 45°from the passing axes of the polarization beamsplitter 410. Thisalternative design is shown in FIG. 5.

TABLE III Length of Lyot depolarizer Linewidth Coherent length Length ofGP depolarizer (□n = 0.2) 1 nm 2.4 mm 1.2 mm 12 mm 0.1 nm 24 mm 12 mm120 mm 0.01 nm 240 mm 120 mm 1200 mm

If the input fiber coupled to the PBS 410 is implemented with a singlemode fiber, the device in FIG. 4 or FIG. 5 can be used as a differentialgroup delay (DGD) line for PMD compensation because differentpolarization components undergo different optical path delays. Therelationship between the minimum device length (excluding lengths of PBSand reflection prism) and DGD is listed in Table IV. Highly compact DGDcomponent can be made with this approach. In addition, because the lightbeams are propagating in the air and the air has negligible dispersion,the device can be used to produce a pure first order DGD.

TABLE IV Equivalent optical DGD path length Device length 10 ps 3 mm 1.5mm 25 ps 7.5 mm 3.75 mm 50 ps 15 mm 7.5 mm 100 ps 30 mm 15 mm 200 ps 60mm 30 mm

FIG. 6 shows a configuration 600 that is capable of depolarizing lightof a long coherence length. In this configuration, a fiber collimator610 is used to focus the light of “p” polarization into a single modefiber 620 as an optical path between the PBS 410 and the reflector 430.A 90-degree Faraday mirror is formed by the reflector 430 and a45-degree Faraday rotator 441 at the other end of the fiber 620 toreflect the light back onto the polarization beamsplitter 410. Due tothe unique ortho-conjugate property of the Faraday mirror, the reflectedlight is orthogonally polarized with respect to the forward propagatinglight everywhere along the fiber 620, despite of the birefringence inthe single mode fiber 620. Consequently, the reflected light becomes a“s” polarized light at the PBS 410 and is reflected towards the outputto combine with light that travels in the short path formed by thereflector 420. Different from the polarization rotator 441 which must bea 45-degree Faraday rotator right in front of the reflector 430, thepolarization rotator 442 may be located at the PBS 410 and may be eithera 45-degree Faraday rotator or a quarter-wave plate. The single modefiber 620 can be easily and inexpensively used as a long optical pathwith a length up to many kilometers. Therefore, with this configuration,the device 600 can depolarize light with a extremely long coherencelength.

Assuming the transmission coefficients of the two paths are T₁ and T₂respectively, the orientation angle α of the input SOP should beα=tan⁻¹(T₁/T₂) in order to have an equal power or the least DOP at theoutput.

In principle, a polarization combiner may operate as a polarizationdepolarizer by combining two independent lasers of orthogonal SOP into asingle beam. However, the optical powers of the two lasers should beequalized in order to obtain small enough DOP. Equalization of the twolasers may require actively monitoring the laser powers and feedbackcontrol the power of one of the lasers, resulting in an increased systemcost.

FIG. 7A shows a device 700 that combines a polarization beam combinerwith a depolarizer. Using this device 700, two laser beams 1 and 2 maybe easily combined with a minimum DOP, without the need to equalize thepowers of the lasers that generate the laser beams 1 and 2,respectively. A PM dual fiber collimator 710 is used to collimate thelaser beams 1 and 2 carried by two input PM fibers with orthogonalpolarizations. A polarization beam combiner 720, such as a Wollastonprism, is used to combine the orthogonally polarized beams 1 and 2 fromthe two input PM fibers respectively carrying the beams 1 and 2 into asingle input beam. A fiber collimator 730 may be used to receive andcollimate the depolarized output beam from the prism reflector 450. Asillustrated, the Wollaston prism 720 includes two birefringent prismsthat are either spaced or cemented to combine two beams with mutuallyorthogonal polarizations into one beam. One used in the reversedirection, a single unpolarized beam can be separated into two divergingbeams with mutually orthogonal polarizations.

FIG. 7B shows relevant states of polarization in the device 700, wherethe collimator 710 and prism 720 are so oriented that both polarizationsare 45° from the passing axis of the PBS 410. In order for the opticalpowers at the output fiber from the short path and long path to be thesame, the mirror 420 in the short path may be slightly mis-aligned toaccommodate for the slightly higher loss in the long path.

An alternative configuration is shown in FIG. 8. In this configuration,a cube polarization combiner 830 is used to replace the Wollaston prism720 in FIG. 7A to combine two input beams into one input beam to the PBS410. As illustrated, an optional half wave plate 840 may be used torotate the two input polarization states so that they are 45° from thepassing axis of the polarization beam splitter 410. The rest of thearrangement is identical to that in FIG. 7A.

A fiber pigtailed polarization beam combiner (PBC) 910 may be cascadedwith the depolarizer depicted in FIG. 4 or FIG. 5 to form aPBC/depolarizer, as shown in FIG. 9. In this configuration, the fiberpigtails of the PBC should be of PM fiber and the PM fiber collimatorshould be rotated such that the slow axis is 45° from the passing axisof the PBS.

Based on the above designs, fiber-coupled devices may be made to havespecifications listed in TABLES V and VI.

TABLE V Specification of an exemplary depolarizer Insertion loss <0.75dB Operation linewidth <0.1 nm Return loss 50 dB min. Center wavelength1550 nm, 1310 nm Wavelength range +/−100 nm DOP <5% Input fiber PM forpolarization sensitive version SM for polarization insensitive versionOutput fiber SM Operation temperature 0 to 60 degree C. Storagetemperature −40 to + 80 degree C. Power handling >300 mW

TABLE VI Specification for an exemplary first order DGD device Insertionloss <0.75 dB 1^(st) order DGD 12 ps, 25 ps, 50 ps, 86 ps Return loss 50dB min. Center wavelength 1550 nm, 1310 nm Wavelength range +/−100 nmHigher order PMD <10 ps² Input fiber SM Output fiber SM Operationtemperature 0 to 60 degree C. Storage temperature −40 to + 80 degree C.Power handling >300 mW

In the devices shown in FIGS. 4, 6, 7A, 8, and 9, the position of one ofthe reflectors 420 and 430 may be adjusted to vary the relative opticaldelay between the two reflected optical signals from the reflectors 420and 430 to produce a variable DGD. In the device in FIGS. 6, 7A, and 8,the fiber 620 may be engaged to a fiber stretcher or other device tovary the fiber length to produce the desired variable DGD.

FIG. 10 shows yet another design 1000 for implementing depolarizers orDGD generators based on optical delay in two different optical paths.The design 1000 includes two optical paths formed by two Faradayreflectors 1050 and 1060 with two single-mode fibers 1040 and 1070,respectively. An input PM fiber 1010 is used to carry the input opticalsignal. The input PM fiber 1010 is oriented such that the incoming lightis split 50% each into the two single mode fibers 1040 and 1070 ofdifferent lengths. As illustrated, a polarization beam splitter 1030 isengaged to the input PM fiber 1010 to perform this power splitting toproduce two orthogonally-polarized beams that are respectively coupledinto the fibers 1040 and 1070. At the opposite ends of the fibers 1040and 1070 are coupled with the Faraday reflectors 1050 and 1060,respectively. Each Faraday reflector may be formed by a 45-degreeFaraday rotator and a reflector. The reflected signals are directed backto the polarization beam splitter which now operates as a polarizationbeam combiner in this reversed input condition. The two reflectedsignals are combined into a single output beam. On the other side of thesplitter 1030, a single-mode output fiber 1020 is coupled to receive theoutput beam.

The device 1000 may be used as a variable DGD generator where amechanism is implemented to adjust the difference in the optical pathlengths of two fibers 1040 and 1070. A fiber stretcher, for example, maybe engaged to one fiber to change the difference. When used as adepolarizer, the difference is set to be greater than the coherencelength of the input signal.

Only a few implementations are disclosed. However, it is understood thatvariations and enhancements may be made without departing from thespirit of and are intended to be encompassed by the following claims.

1. A device, comprising: a polarization beam splitter (PBS) to receivean input optical beam with a coherent length and to split the inputoptical beam into a first beam and a second beam polarized orthogonal tosaid first beam; a first reflector positioned relative to said PBS toreflect said first beam back to said PBS; a first polarization elementlocated between said first reflector and said PBS to rotate apolarization of a reflection of said first beam at said PBS to beperpendicular to said first beam when initially exiting said PBS; asecond reflector positioned relative to said PBS to reflect said secondbeam back to said PBS; and a second polarization element located betweensaid second reflector and said PBS to rotate a polarization of areflection of said second beam at said PBS to be perpendicular to saidsecond beam when initially exiting said PBS, wherein said first and saidsecond reflectors are positioned to produce a difference in opticalpaths of said first and said second beams upon being reflected back tosaid PBS that is greater than said coherent length and that light ofsaid first beam and light of said second beam upon being reflected backto said PBS are not coherent to each other and combined light producedby said PBS from the light of said first beam and the light of saidsecond beam upon being reflected back to said PBS is depolarized.
 2. Thedevice as in claim 1, wherein said first polarization element is aquarter wave plate.
 3. The device as in claim 1, wherein said firstpolarization element is a 45-degree Faraday rotator.
 4. The device as inclaim 3, wherein said 45-degree Faraday rotator is located in front ofsaid first reflector to form a Faraday reflector.
 5. The device as inclaim 4, further comprising a fiber coupled between said PBS and said45-degree Faraday rotator.
 6. The device as in claim 5, wherein saidfiber is a single-mode fiber.
 7. A device, comprising: a polarizationbeam splitter (PBS) to receive an input optical beam with a coherentlength and to split the input optical beam into a first beam and asecond beam polarized orthogonal to said first beam; a first reflectorpositioned relative to said PBS to reflect said first beam back to saidPBS; a first polarization element located between said first reflectorand said PBS to rotate a polarization of a reflection of said first beamat said PBS to be perpendicular to said first beam when initiallyexiting said PBS; a second reflector positioned relative to said PBS toreflect said second beam back to said PBS; a second polarization elementlocated between said second reflector and said PBS to rotate apolarization of a reflection of said second beam at said PBS to beperpendicular to said second beam when initially exiting said PBS,wherein said first and said second reflectors are positioned to producea difference in optical paths of said first and said second beams uponbeing reflected back to said PBS that is greater than said coherentlength; two fibers to carry two input beams with orthogonalpolarizations; a dual fiber collimator coupled to said two fibers tocollimate said two input beams from said two fibers; and a polarizationbeam combiner to combine said two input beams into a single beam as saidinput optical beam to said PBS.
 8. The device as in claim 7, whereinsaid two fibers are polarization maintaining fibers.
 9. The device as inclaim 7, wherein said polarization beam combiner is a Wollaston prism.10. The device as in claim 7, wherein said first polarization element isa quarter wave plate.
 11. The device as in claim 7, wherein said firstpolarization element is a 45-degree Faraday rotator.
 12. The device asin claim 11, wherein said 45-degree Faraday rotator is located in frontof said first reflector to form a Faraday reflector.
 13. The device asin claim 12, further comprising a fiber coupled between said PBS andsaid 45-degree Faraday rotator.
 14. The device as in claim 13, whereinsaid fiber is a single-mode fiber.
 15. A device, comprising: apolarization beam splitter (PBS) to receive an input optical beam with acoherent length and to split the input optical beam into a first beamand a second beam polarized orthogonal to said first beam; a firstreflector positioned relative to said PBS to reflect said first beamback to said PBS; a first polarization element located between saidfirst reflector and said PBS to rotate a polarization of a reflection ofsaid first beam at said PBS to be perpendicular to said first beam wheninitially exiting said PBS; a second reflector positioned relative tosaid PBS to reflect said second beam back to said PBS; a secondpolarization element located between said second reflector and said PBSto rotate a polarization of a reflection of said second beam at said PBSto be perpendicular to said second beam when initially exiting said PBS,wherein said first and said second reflectors are positioned to producea difference in optical paths of said first and said second beams uponbeing reflected back to said PBS that is greater than said coherentlength; two fibers to carry two input beams with orthogonalpolarizations; and a polarization beam combiner to combine said twoinput beams into a single beam as said input optical beam to said PBS.16. The device as in claim 15, wherein said first polarization elementis a quarter wave plate.
 17. The device as in claim 15, wherein saidfirst polarization element is a 45-degree Faraday rotator.
 18. Thedevice as in claim 17, wherein said 45-degree Faraday rotator is locatedin front of said first reflector to form a Faraday reflector.
 19. Thedevice as in claim 18, further comprising a fiber coupled between saidPBS and said 45-degree Faraday rotator.
 20. The device as in claim 19,wherein said fiber is a single-mode fiber.